Tensile test ISO 6892 – Knowledge
Tensile strength, elongation, yield strength – simply explained
Really everything that is manufactured must (should) be tested so that the material or product can fulfil its intended purpose. This could be the tab on a drinks can that breaks without the can being opened, or a vehicle body that turns into a “banana” because the steel used was defective. This applies to all materials: Plastic, adhesives, glass and especially metal. As soon as a product has to withstand forces, steel is usually the material that best fulfils this task. NOT the tensile strength (maximum force divided by the cross-section of the sample) is the most important characteristic value but rather the damage limit ReH (yield strength) or the equivalent yield strength = proof strength Rp0,2.
Here you will find information about the difference between Reh and Rp0,2, other material properties and a Tensile test required Tensile testing machine.
Interesting facts
The requirements for metals are very different, sometimes extreme. Metals for steel springs must be extremely strong/hard and yet elastic/springy. On the other hand, the production of cooking pots, for example, requires that a flat sheet of metal be extremely deformed (deep drawing = material stretching) without thinning or cracking.
In order to exclude production-related quality fluctuations, metal intermediate products are regularly subjected to a tensile test on a Tensile testing machine checked. The Rods, pipes, sheets require the production of different tensile specimens: cylinder head, threaded head or flat sheet metal specimen.
The most important mechanical technological material characteristics of a metal tensile test are:
- elastic modulus / auxiliary line Hooke's line
- Stretch limit Deer / yield strength IDR 0,2
- Tensile strength Rm
- Lüders, Gleichmaß, Bruch strain A5,65/A80
- constriction Z
The requirements for metals are very different, sometimes extreme. Metals for steel springs must be extremely strong / hard (high tensile strength) and still elastic / springy (elastic stretch) On the other hand, the production of cooking pots, for example, requires that a flat sheet of metal is extremely deformed (deep drawing = material deformation or plastic strain) without causing thinning or cracking.
In order to exclude production-related quality fluctuations, metal intermediate products are regularly subjected to tensile testing on a Tensile testing machine tested. The bars, tubes and sheets require the production of different tensile specimens with cylinder head, threaded head or flat sheet specimen.
examination procedure
The tensile test according to ISO 6892 is carried out classically in three phases:
- Elongation of the tensile specimen at low speed in the elastic range
Elastic, reversible stretching - Constant creep rate between elastic and plastic strain
First damage: Yield strength ReH / Proof strength Rp0,2 - Significant increase in speed in the plastic range up to fracture
Irreversible stretching
Required for a tensile test ISO6892
Tensile testing machine
Class 05 ISO7500
testing software
metal tensile test
Scheme tensile test
Tensile strength is NOT the most important result! Linear stress increase to zigzag curve:
What happens at the yield strength ReH?
The first damage to the metal
More important than the tensile strength is the yield strength ReH (or yield strength Rp0,2): As soon as metal is permanently deformed, this is the “beginning of the end”.
Low-alloy or unalloyed iron has a yield strength ReH. When it is stretched in a tensile test, a force or the stress to be determined causes spontaneous yielding. The most heavily loaded force associations of the "hooked" metal crystals slip off and a clearly visible drop in stress occurs. After that, other "Velcro hooks" allow a renewed increase in force until they too fail and slip off. Once all the coarse hooks have slipped off, the effect disappears and all the crystals are stretched together until the sample breaks.
Metallic structure
Polished + etched: The crystals / grain boundaries are visible
microstructure analysis: A metal sample is separated, ground, polished, etched and examined under 500x magnification using a lot of cooling water.
What is the purpose of the yield strength Rp0,2?
If iron is alloyed (chromium, manganese, etc.) steel is produced. This results in a finer crystal structure. This and cold forming (rolling, hammering) or heat treatment suppress the Lüders effect.
Instead of the easily evaluable yield strength ReH, the Young's modulus and, in parallel, the "equivalent yield strength" must be determined. To do this, the plastic yield strength Rp0,2% is evaluated parallel to the Hooke's line.
Tensile test – part of quality assurance
Materials can now be produced in industry with a high degree of precision to the desired material quality. Nevertheless, the process of material production and further processing up to the finished end product (material - not component) must be constantly monitored, as a variety of factors can have a strong influence on quality. For example, in the steel industry, in the liquid phase of pig iron / crude steel, the composition of the alloy is almost the only criterion. Here, for example, spectral analysis is used to determine the various elements, including trace elements. During further processing, however, the various processes sometimes have extreme influences on the material. The properties of the material can be changed extremely by a variety of treatment methods. Here are a few processing methods as an example:
- Rolling the steel to change its shape/thickness – Compaction of the material = higher tensile strength and hardness, lower elongation
- hardening of the steel – Structural change = higher tensile strength and hardness, lower elongation, embrittlement
- galvanizing – Material application = prevents rusting, chemical treatment in several steps can lead to embrittlement
- intermediate annealing – Structure relaxes = tensile strength + hardness decrease, elongation increases, embrittlement decreases
- blasting of the material (with steel ball, ceramic balls, sand) – material removal (rust) / surface compaction = tensile strength / hardness on the surface increases
A variety of further treatment and processing steps can dramatically change the material properties. However, if the design of a component or, for example, a bridge requires the use of steel with a strength of 800 MPa, then it must also be checked that this requirement is met. Failure of the component must be ruled out (bridge construction, vehicle bodies, cranes and all assemblies and components that have to bear static or dynamic loads).
Here are diagrams that show the difference between a yield point and a proof point (left: diagrams with yield point / right: diagrams with a proof point)
Determination of material properties of metallic materials
There is no such thing as "good" or "bad" metal, but rather the intended use/cost determines the requirements for the material. For example, steel for a valve in an internal combustion engine must be extremely high-strength and hard to prevent premature wear. On the other hand, sheet metal for a car fender must be relatively soft so that the material can be formed without cracking/breaking (deep-drawing process).
The material properties are determined by testing a tensile specimen using a metal tensile test. These tests are most commonly carried out in the steel industry, plastics industry and rubber industry (and some more).
Depending on the type of use, the products (base materials) are processed in different geometries (pipes, bars, slabs, sheets and of course entire components). This means that a wide variety of sample shapes are required in order to be able to determine the material characteristics. If the product has a large volume, in the steel industry a tensile test specimen with a round cross-section is often made from the solid material (lathe). Other products (e.g. sheets) require that a flat tensile test specimen with a square or rectangular cross-section is produced and tested.
So that the results can be compared with each other regardless of the cross-section, all measured values are related to a comparison geometry. This is usually mm² for the strength or a reference length L0 (length zero - technical term: initial measuring length) for the elongation. For example, the tensile strength (abbreviation Rm) is given in the unit N/mm² (Newton per mm²) or in accordance with ISO 6892 in MPa (Mega-Pascal = conversion factor 1:1 to Newton mm²).
At a
- Plastic the tensile strength is e.g. 20 MPa (for illustration: ~ 2 kgf weight per mm²)
- soft sheet metal (car fender) at 200 MPa (for illustration: ~ 20 kgf weight force per mm²)
- high-strength spring steel at 2.100 MPa (for illustration: ~ 210 kgf weight force per mm²)
In order to be able to evaluate this unity, the following should be said for the “average citizen” to illustrate this:
1000 MPa corresponds to approximately 100 kgf weight force (approx. 101,93 kg – divisor 9,807)
If a square wire with a cross-section of 1 x 1 mm = 1 mm² is loaded with a force of 1000 Newtons (related to 1 mm² = 1000 MPa) or 101,93 kg, it will stretch and break. The test specimen is clamped in a grip and subjected to very slow, steadily increasing extension. The test is now recorded electronically in the form of a diagram. In today's electronic evaluation systems, however, the diagram is only used for visual control and representation of the test progress. The parameters are determined using complex algorithms. However, the diagram representation and the results should and must be approximately identical in order to provide the user with a comparison between measured value <-> diagram and to enable a visual assessment.
Basics of the following diagrams
Characteristic data with R (stress values σ) indicate the force currently applied at this point divided by the initial cross-sectional area (S0) of the sample – shown / “read” on the vertical axis in the diagram.
Characteristic data with A (strain values ε) indicate the current extension (elongation) of the test specimen at this point in relation to a starting point (L0 – initial measuring length) – shown / “read” in the diagram on the horizontal axis of the graph
The simplest form of explanation is usually the representation using sketches. Below is a sketch of a graph of yield strength material
Explanation of the material characteristics
- ReH – Upper yield strength
If the test specimen is subjected to further stress, the first damage occurs: the force groups in the material fail (in simple terms: fibers tear) and there is a spontaneous drop in force (or drop in stress). This highest point on the line is referred to as ReH. After this first damage, the specimen lengthens irreversibly - permanently. The designer could load a material (component) up to the ReH value without causing permanent damage. Of course, the designer stays well away from this point to be on the safe side. - ReL – Lower Yield Limit
After the first damage to the material has occurred (ReH), the force/stress (depending on the material) drops significantly. This sometimes occurs to a much lower value than is usual for the Lüders strain. The first (very deep) drop in stress is called the transient response (extreme downward deflection of the curve directly after ReH). This deflection is ignored for the ReL parameter. In the Lüders area that follows, the lowest deflection of the graph is sought. This point is called the lower yield point and is the point to which the stress drops to a maximum before a further steady increase in stress occurs. - hardening of the material (steady increase after the Lüders strain)
After all the "problematic" forces have broken (Lüders stretching is complete), the tension increases again and steadily. This can be explained as follows: individual fibers in a rope were too short. These have now broken (Lüders behavior). After all the "too short fibers" have broken, all fibers of a rope take on the load again - the tension increases steadily again (without any more individual fibers breaking). - Rm – tensile strength
After the tension has continued to rise steadily, a state is reached in which no further increase in force is required to lengthen the tensile test specimen: The material continues to stretch evenly until the test specimen begins to constrict (waist) at one point (initially slightly). The highest point of the force (tension) is determined and the tensile stress is determined (maximum force divided by sample cross-section = Rm in N/mm² or correct MPa). After the maximum force is exceeded, the sample begins to constrict at one point. This constriction now occurs more and more quickly until the test specimen breaks and is split in two. - Lüders – strain Ae (not marked with a letter in the diagram)
In the next part of the tensile test, these stress drops occur with varying frequency (depending on the material properties): An oscillation curve is created which is called Lüders strain after its discoverer. The crystals/groups of forces slide past each other (stress drop) and then get caught again (stress increase). The easiest way to imagine this is how the individual fibers move when you look at the fibers of a rope (more below). - Elastic strain (extension of the sample to ReH)
- Plastic strain (extension of the sample after ReH or after leaving the straight line)
As soon as the yield point ReH is exceeded or the straight line of elasticity is left, permanent plastic strain begins. The sample is permanently elongated and does not return to its original length even if the force is completely removed. - Elongation at break A
This is the permanent strain that the sample experiences until it breaks. As soon as the sample breaks, the elastic part contracts again and shortens the sample slightly (typically 0,3%). - Total elongation at break At
Is the permanent strain that the sample experiences until it breaks plus the elastic part. This strain can only be measured if a strain gauge records the elongation of the sample (permanent strain including elastic component) until fracture. - uniform strain Ag
The standard tensile specimen stretches evenly in the parallel area of the cross-section. A standard tensile specimen usually has a dumbbell shape (bone shape). The uniform elongation Ag is the elongation of this specimen from radius to radius up to the maximum force. - Total uniform strain Agt
Like Ag – but including the elastic part (the elastic part contracts again after fracture)
material without a pronounced yield point
This material is either an iron material that has undergone further treatment (cold forming, heat treatment, etc.) or a steel (iron with an additional alloy). In these materials, the yield strength known from iron is no longer present, and alloyed steel and other metals (aluminium, bronze, etc.) have no yield strength. The material is constantly extended during a tensile test without a spontaneous drop in force (drop in stress). Permanent extension (damage) to the material is therefore not as obvious as in the first diagram. Thus, an equivalent yield strength - the proof strength - was defined in the standard as a way of determining the first damage. It is observed at which force or stress the material loses its elasticity and is permanently deformed (stretched). As a comparison standard, it was generally agreed that an extension of 0,2% permanent elongation should be used.
Hooke's line
Robert Hook discovered that every body exhibits elastic behavior. If you extend a body by a not too great length, it contracts again. This is most clearly observed with rubber or a spring.
elastic modulus
The elastic modulus is the calculation of the material characteristic value of the material elasticity value discovered by Hook. The simplest way to explain this function is as the spring constant of the material. If you stretch a spring, you have to use a certain force per mm to extend the spring. For example, you need 100 N (approx. 10 kg) to extend the spring by 1 mm. The spring constant C = 100 N /mm. The elastic modulus is calculated in a similar way. However, the forces are replaced by stresses and the lengths by strains. The nominal elastic modulus of a material, e.g. steel (depending on the alloy, e.g. 210.000 MPa) results from the following theoretical calculation: If you could double the initial length L0 without the steel being permanently extended (damaged) (without the curve tipping horizontally), the material would reach a stress of the aforementioned 210.000 MPa. This is certainly not possible because any steel would be plastically deformed for a long time and would certainly break if it were extended by 100%. But the Young's modulus is just a theoretical number. This ultimately expresses the steepness of the Hooke's line (theoretical graph) - the red line in the diagram. The Young's modulus must be determined in order to be able to apply an offset of 0,2% (or other) to this straight line in order to then be able to tap the stress at the intersection point of these parallel lines.
yield strength Rp0,2
Stress value that is determined as soon as the sample has been permanently extended by 0,2% strain. For this purpose, a straight line is drawn parallel to the modulus of elasticity. The stress at 0,2% strain is measured at the intersection point with the graph.
yield strength Rp0.01
Like Rp0,2 but at 0,01% extension. This is the smallest usual yield strength and is used, for example, in mechanical engineering for static calculations. A structure may only be loaded to such a high degree that it is never permanently deformed. So an even smaller value of the permanent deformation is of interest for the static calculation. For the evaluation, a straight line is drawn parallel to the modulus of elasticity. The stress at 0,01% extension is taken at the intersection point with the graph.
yield strength Rp1.0
This yield strength is less common. It corresponds to the stress at a permanent elongation of the specimen by 1.0% strain.
comparability of elongation at break
In common parlance, stretching refers to experiences and observations from our daily lives. Since we can only observe this phenomenon in very elastic, highly stretchable materials, this term is associated with an extension that refers to materials such as rubber. The observation leads to the conclusion that all areas of the (rubber) tensile test specimen extend evenly until it breaks. Theoretically, one can subject a parallel strip or round bar to a tensile load and at any time during the tensile test, measure the same extension at any point (uniform extension until it breaks).
- Rubber remains completely elastic until it breaks and – as long as it is not torn – it will contract completely to its original length.
Metal behaves differently: once a damage threshold is exceeded, a permanent extension occurs (even if no damage / constriction is visible). As soon as the sample has been extended by 1 mm, for example, this extension only contracts by 0,3 mm - 0,7 mm extension remains. - Initially, a steel sample actually elongates evenly at all points (of a homogeneous cross-section) (uniform elongation). However, this behavior changes dramatically. As soon as all structural components have been elongated to the maximum, further flow only occurs partially. Where the fracture occurs later, the material flows disproportionately and a constriction occurs (waist formation). In the area of the later fracture, flow behavior (elongation) occurs, whereas other areas are no longer elongated.
- Strain depends on the cross-section or volume:
This becomes clear using the example of balloons:
A small balloon can only be inflated slightly before it bursts.
A large balloon can be inflated accordingly larger.
Steel: A sample with a large cross-section has more material inside to flow than a sample with a small cross-section. Therefore, a sample with a large cross-section (with the same sample or initial length) can be extended further than a sample with a small cross-section:
A round rod Ø 10 mm can be cut from 100 mm to 120 mm can be extended.
A round rod Ø 20 mm can be cut from 100 mm to 125 mm can be extended (+ 5 mm).
To the to make elongation at break comparable A system had to be integrated into the expansion calculation to take the different volumes into account. Since metal flows proportionally in relation to volume (more volume = greater extension), this law was taken into account. The reference length Lo (zero length) is therefore constantly recalculated based on the cross-section by including the proportionality factor. So it is not tested with a rigid Lo, but the zero length (Lo ) is calculated by including the proportionality factor K:
Lo = K x (root So)
K = proportionality factor 5,65
So = sample cross-section in mm²
Calculation of Lo (initial length)
Round sample Ø 10 mm: Lo = 5,65 x (Root(10 x 10 x 3,14 /4)) = ~ 50,06 ~ 50 mm
Round sample Ø 12 mm: Lo = 5,65 x (Root(12 x 12 x 3,14 /4)) = ~ 60,07 ~ 60 mm
In addition to the proportionality factor K 5,65, there is also the less common factor 11,3.
A probe arm strain gauge or long-stroke strain gauge simplifies the testing and evaluation of the elongation at break: Its cutting edges contact the sample at exactly this measurement (50,0 mm or other, non-rounded numbers) and follow this initial measuring length (Lo) until breakage. The calculation can then be carried out in a simple manner because only the measured values of the probe arm strain gauge are included in the measurement.
If no (cost-intensive) long-distance strain gauge (probe arm strain gauge) is used, the sample must be marked before the test at a grid spacing of 5 mm or with punch marks (which do not provoke premature fracture at this point). After the fracture, the fragments are placed together and the extended initial measuring length is measured.
Special features of thin sheets
If sheets are too thin, no material can flow out of the thickness: The sample expands not proportional to the cross-section. For sheets less than 3 mm thick, the elongation is therefore determined on a "non-proportional flat sample" with a rigid reference length. For iron and steel sheets, the elongation calculation is based on an initial length Lo of 80 mm (A80 elongation). For non-ferrous metals, the elongation calculation is based on a Lo of 50 mm (A50 elongation).
Stretching is not the same as stretching
If the result of the elongation at break does not contain an index (i.e. only capital letter A), one must assume a flow behavior proportional to the volume. In this case, it is the elongation at break A(5,65).
If the flow behaviour is NOT PROPORTIONAL, an index must be added to the elongation value: This is a non-proportional flat sample A80 or A50 (or the proportional flow behavior A11,3)
So if you want to compare strains, you have to consider an index (if necessary). If the strains of different samples are to be compared, they have to be converted. There are calculation formulas for this, or we have created a website that converts the strain into other types of strain.
calculation of elongation at break
(Illustration: sample extension in mm)
The graphic shows why the elongation at break cannot be measured over the traverse path: The elongation values result from a total of 3 areas:
- elastic strain (numbers 0,05 mm), are subtracted at the end because the sample contracts around this part after the fracture
- plastic strain in the entire parallel area Lc (uniform strain Ag to Fm)
- Yield strain in the area of the constriction – the extension only occurs in the yield area
Only the numbers in the Lo range may be included in the elongation at break calculation!
Calculation of elongation at break
From our experience, we know that calculating the elongation (flow behavior) of metals / a tensile test sample is the most difficult characteristic to explain. Metals have a special elongation characteristic. We have all observed how metal is permanently deformed: if you bend a sheet of metal just a little, it springs back to its original shape (elastic behavior). If you keep bending, however, you will eventually reach the point where this piece of metal remains curved: it is permanently deformed. Steel behaves under tensile load in a similar way to the elastic / plastic bending of steel. If you stretch this steel just a little, it contracts again (similar to rubber). The steel only stretches permanently when you exceed the damage limit. For certain tasks in industry, it is important to determine the elongation behavior of a metal precisely. Only if you know the forming behavior (elasticity) exactly can you ensure that the metal can be formed into the desired shape without causing too much damage (premature breakage). This becomes logical once you understand that a cooking pot is created from a flat sheet of metal through extreme forming - a deep-drawing process with extreme elongation.
Metal stretches until it breaks, but not evenly at all points. As soon as the uniform stretch at the maximum force is exceeded, the material flows more and more extremely at the weakest point (and only there), constricts (waist formation) and breaks there. Since the flow behavior of the metal is not the same at all points, a system must be found to be able to determine the stretch in a comparable way. Therefore, the stretch A (breaking stretch) is only related to a precisely defined but variable distance. This reference value (zero length - L0 for short) is used as the basis for the calculation and is calculated using the formula.
Note: The strains cannot be directly compared. A strain A (A5,65) differs considerably from a strain A80 / A11,3 / A50 / A100 / A200. To compare the strains, they can be converted.
A Elongation at break (in %)
L0 Initial measuring length of the sample (initial length)
Lu length after fracture (includes initial measuring length L0 and the extension of this part)
Formula for calculating the elongation at break A for a tensile test
(Note: If the short letter A is used without an index, it is the extension A5,65)
Formula:
A = ((Lu – L0)/L0)x 100 or:
Definition of tensile testing for metals
|
abbreviation |
unity |
designation / simplified declaration |
|
a0 |
mm |
Initial thickness of a flat specimen or wall thickness of a pipe at the beginning of the tensile test |
|
b0 |
mm |
Width of a flat specimen in the test length | average width of a pipe strip specimen | profile wire at the beginning of the tensile test |
|
D0 |
mm |
outer diameter of a pipe |
|
L0 |
mm |
Initial measuring length (reference length or initial length for the elongation) |
|
LC |
mm |
test length (parallel part of the measuring length between the radii) |
|
Lt |
mm |
Total length of the sample including heads |
|
LU |
mm |
Measuring length after fracture (L0 stretched) |
|
S0 |
mm² |
Initial cross-section of the sample: Thickness x width | Diameter | Pipe diameter x wall thickness | Weight method: Weight / Length / Density |
|
SU |
mm² |
smallest cross-section after fracture (for calculating the necking – flow behavior / strain behavior in the cross-section) |
|
K |
- |
Proportionality factor – ratio of the cross section to L0 (5,65 or 11,3) |
|
Z |
% |
Fracture area – relationship between S0 and S.U(Calculation of the necking – flow behavior / strain behavior in the cross section |
|
R(σ) |
MPa |
Stress – force divided by the initial sample cross-section |
|
A |
% |
Elongation at break – extension of the sample relative to the initial measuring length L0 (without index = elongation A5,65) |
|
At |
% |
like A, but this value also includes the (total) elongation including elastic part |
|
Ag |
% |
Uniform elongation: The elongation of metals occurs up to the force maximum (Fm Force Maximum) or stress maximum (Rm) |
|
Agt |
% |
like Ag, but this value includes the elastic strain |
|
A5,65 |
% |
typical proportionality factor for calculating the L0 (not for flat samples thickness <3,0 mm | wires Ø <4 mm applicable) |
|
A11,3 |
% |
proportionality factor for calculating the L0 (not applicable for flat tensile specimens thickness <3,0 mm | wires Ø <4 mm) Note: Elongation values are not directly comparable with each other – please use our calculator to convert. |
|
A50 |
% |
Elongation at break of flat tensile specimens made of sheet metal with a thickness of 0,1 – 3,0 mm (most common specimen shape for non-ferrous metals such as aluminum, copper |
|
A80 |
% |
Elongation at break of flat tensile specimens made of sheet metal with a thickness of 0.1 – 3.0 mm (most common specimen shape for steel flat tensile specimens |
|
A100 |
% |
Elongation of wires at break: Wires often break at an undefined point. If one were to take an A5,65 elongation |
test report (test certificate)
Content reproduction of ISO 6892-1 in relation to a test report (see also below: PDF download test report)
ISO 6892-1 from 2017-02 requires that the test report must contain the following information (exception: the parties agree otherwise)
| 1. Reference to the ISO 6892 standard and the test speed -here: A224 |
A2XX = 0.00025 mm/mm/second (reference value extensometer) AX2X = 0.00025 mm/mm/second (reference value traverse travel related to Lc) Axx4 = 0.0067 mm/mm/second (reference value traverse path related to Lc) |
| 2. Identification of the sample | sample number for traceability |
| 3.Material, if known | ST370 – structural steel |
| 4. Type of sample | flat sample |
| 5. Sample position and direction | Sampling: 50 mm from the strip edge. Transverse sample |
| 6. Test control types + speeds | only if deviation, see also 1. |
| 7. Test results | Reh, Rp0,2, Rm, Ag, A80 (E-modulus is NOT a result according to ISO6892-1 but serves as a good result of the tests Values rounded R-values without decimal places to the nearest whole number A-values rounded to 0,5% Z-fraction area rounded to 1% (if relevant) |
